A multi-chip module ("MCM") consists of a number of unpackaged chips which are mounted directly on and electrically interconnected by a passive substrate, thereby enabling high density packaging. The chips in a MCM are usually active integrated circuit chips. However, it is not always possible to implement a desired module function exclusively with integrated circuit chips. Specifically, there is often a need for discrete resistors. In digital circuits, discrete resistors may be needed as transmission line terminators, pull-ups or pull-downs for die outputs, and tie-ups or tie-downs for die inputs. In analog circuits, discrete resistors may be needed for operational amplifier networks.
An unpackaged, single resistor, also known as a "single resistor chip," is available and can be mounted on the substrate together with the active chips. The single resistor chip, however, is highly undesirable. Small in comparison to the active chips, the single resistor chip still consumes inordinate amounts of substrate surface area and is much more difficult to assemble. It may also introduce noticeable parasitic capacitances and inductances.
In the past, attempts have been made to mitigate the disadvantages of the single resistor chip by "sweeping" a number of single resistors together to form a multi-resistor chip. This leads, however, to a different problem. Whenever the resistors in a given multi-resistor chip require different values because of a specific electrical application, a standard "off-the-shelf" chip in which all the resistors have equal values can no longer be used.
The need for resistors having differing values of resistance in a multi-resistor chip may occur even if the application calls for equal resistances. In the case of pull-down resistors or transmission line terminators, for instance, an application may need a large number of equal resistors. However, if the resistors or terminators are concentrated in a multi-resistor chip, each resistor has to be connected by a trace on the substrate to connection points which then invariably adds the resistance of the trace itself to the resistance of the individual resistor. Thus, the resistance of each individual resistor on a multi-resistor chip may have to be individually adjusted in order to compensate for the trace resistance to obtain the required total resistance.
Another approach previously used to mitigate the single resistor chip problem has been to integrate the resistors as thin-film resistors into the substrate. The conventional method of constructing thin film resistors involves creating a horizontal rectangle of resistive material located within a film (usually by etching or a similar process). Disadvantages of the thin film approach include the need for additional steps and the introduction of different materials to the substrate manufacturing process. The thin film resistors may also consume significant amounts of substrate area, whereby reducing the area available for the necessary interconnect traces.